The determining of chemical oxygen demand (COD) plays an important role in the investigation especially of aqueous samples and the evaluation of waste water quality. This is true, for example, in the control of waste water treatment in a clarification plant or in the monitoring of waste waters both in the case of introduction of such into other waters, as well as also at the inlet or outlet of clarification plants.
In the case of known wet chemical methods for determining the COD value, oxidation of the organic materials dissolved in a liquid sample occurs by chemical reaction with an added oxidizing agent, e.g. K2MnO4 or K2Cr2O7. For masking disturbing ions, e.g. chloride, frequently, a mercury salt is added.
These methods have, in the meantime, been set down in DIN-standards (German Institute for Standardization). Disadvantageous in the case of these methods is that they do not work continuously, since the sample to be examined must, as a rule, be reacted at least one hour long with the oxidizing agent under increased temperature, before the consumption of the oxidizing agent and therefrom COD value of the sample can be ascertained. Furthermore, the used chemicals, especially K2Cr2O7, are poisonous and potentially strongly environmentally damaging, so that, on the one hand, complex precautionary measures must be undertaken, in order that the chemicals not get into the environment, while, on the other hand, a great effort for the disposal of the consumed samples and chemicals must be made.
There are, therefore, a number of approaches for determining chemical oxygen demand without the addition to the sample to be examined of reagents acting as oxidizing agent.
In German Patent, DE 2 135 949 A1, a measuring method is described, in the case of which water electrolysis is applied, wherein the hydrogen developing electrode is separated by a membrane from the sample to be examined, into which the oxygen developing electrode is immersed, wherein via the membrane a charge carrier exchange can occur. The measuring principle rests on the concept that the oxygen amount theoretically produced by the electrical current flow is compared with the actually produced oxygen. In such case, the theoretically produced oxygen amount is ascertained via the used quantity of charge, while the actually produced oxygen amount is measured as a pressure increase. If the actually formed oxygen amount falls behind the theoretically expected value, then the reason for this is either that charge was consumed for direct oxidation of substances contained in water, or the developed oxygen was partially consumed as oxidizing agent. This method requires a relatively complex measurement setup. Furthermore, a continuous measuring is not directly possible with this method.
European Patent, EP 282 441 B1 describes a further electrochemical method for determining COD by unspecific electrochemical oxidation of ingredients dissolved in a water sample at controlled potential, wherein, via a lead dioxide, working electrode, electrical current flowing through the aqueous solution is measured and provides a measure for the quantity of material converted at the electrodes per unit time. The method rests on the concept that on a lead dioxide anode preferably a forming of OH-radicals (OH.) and ozone (O3) occurs, which are two very reactive oxidizing agents, and that their rate of formation and therewith also their consumption caused by oxidation of the ingredients of the sample solution can be registered by measuring the electrical current flow through the lead dioxide electrode.
German Patent, DE 196 37 704 A1 describes a similar method, which is supposed to enable the provision of extensive information concerning the nature of the ingredients present in the sample responsible for the COD. In such case, a three electrode circuit with a lead dioxide electrode as working electrode is used. Basic idea of the method described there is to vary the potential on the lead dioxide electrode while measuring the associated electrical current level arising between the lead dioxide electrode and the counterelectrode and flowing through the sample, or alternatively to vary the electrical current level of the electrical current flowing through the sample and measure the associated potential on the lead dioxide electrode and so to obtain a number of measured values. By evaluation of the measured value curves, for example, by comparison with reference measured value curves for certain typical sample compositions, qualitative information on the presence of slightly or difficultly oxidizable materials in the sample can be made.
A disadvantage of lead dioxide electrodes is that they must be regularly regenerated, since a passivating cover layer can form on them during operation. Furthermore, there is the danger that, due to chemical or mechanical action on the lead dioxide electrode, poisonous lead compounds can get into the sample, e.g. a wastewater or drinking water sample, and possibly escape into the environment.